Osseointegrated implants are routinely utilized in a broad range of oral and extraoral applications including removable and fixed dental prostheses, in re-construction of the head and neck, as a transmission path for bone anchored hearing aids (BAHA™), to provide anchorage in orthodontic treatment and in orthopedic applications. FIG. 1 shows a cross-sectional side view of a typical in-situ implant and abutment system. Such implants are typically 3-6 mm in diameter and range in length from 3-4 mm (BAHA and orbit applications) to 7-20 mm (dental reconstructions). Such implants are often formed of Titanium.
The success of these implants is dependent on the quality of the bone-implant bond at the interface of the implant. A direct structural and functional connection between living bone and the surface of a load-carrying implant is defined as osseointegration. This process typically begins immediately after the implant has been installed. If this does not occur, the development of connective soft tissue in the bone-implant interface may begin and can lead to failure of the implant. The status of the implant-bone interface during this crucial time is extremely important in evaluating when the implant can be put into service (loaded) or whether further healing is necessary.
In addition, over time osseointegration can deteriorate and/or the degree of bone in contact with the implant surface can reduce. Although implant survival rates are high in many applications, it is important to be able to determine if any change in the health of this interface occurs. As a result of these potential clinical conditions, there is an ongoing desire to monitor the “health” or integrity of the bone-implant interface from initial installation of the implant throughout the life of the implant.
Conventional diagnostic techniques, such as radiography and magnetic resonance imaging, are generally able to evaluate bone quantity and in some cases may provide parameters that relate to bone quality (eg. Hounsfield radiodensity scale). However, these techniques are limited in their ability to monitor the actual bone-implant interface, as the implant tends to shield this region resulting in poor image resolution in this vital area. Therefore, the condition of the bone-implant interface including the implant threads and the adjacent tissue undergoing remodelling is much more difficult to evaluate. When using radiography, the changes in bone are often well advanced before becoming evident on radiographic images. Furthermore, images obtained in this manner are costly and high quality radiographs carry additional risks associated with radiation exposure.
Other techniques such as measuring removal torque are too invasive to be used in either the operating room or for clinical visits. As a result, dynamic mechanical testing methods have been proposed and are presently in use. These mechanical techniques are all, in one form or another, based on determining the resonant frequency of the implant-tissue system including the transducer. As the resonant frequency is dependent on the manner in which the implant is supported by the surrounding biological tissue, changes in this resonant frequency (perhaps coupled with changes in the internal damping) should be linked to changes in the status of this interface. This, of course, assumes that there are no other changes in the implant system (such as a loosening of the abutment/implant joint) that may overshadow those in the interface.
Presently, the primary commercially available system developed specifically for monitoring implants is Osstell™, which employs a transducer attached to the abutment or directly to the implant. The transducer excites the system over a range of frequencies while simultaneously monitoring the resulting transducer motion to determine the resonant frequency of the overall implant/transducer or implant/abutment/transducer system. The results of several investigations using this system have reported varying degrees of success in identifying changes in the implant status. A disadvantage of the Osstell is that it is designed to be used with retrievable systems only.
Alternative techniques to the Osstell are based on transient measurements in which the abutment is excited using an external impact. Subsequently, a measurement method was developed that utilised an instrumented impact hammer to evaluate the mechanical impedance variations caused by interface changes. One approach involved an impacting rod to excite the abutment and the resulting resonant frequency was determined from an acoustic signal obtained from a microphone mounted in close proximity.
Another system that has been used is the Periotest™, which was originally developed to measure the mobility of natural dentition. As shown in FIG. 2, there is a Periotest handpiece, which contains a metal rod of approximately 9 grams. The metal rod is accelerated towards the implant-abutment via an electromagnet. The acceleration response of the rod, while in contact with the implant-abutment, is measured using an accelerometer attached to the rear of this rod. In particular, the acceleration signal is used to determine the period of time during which the rod and tooth remain in contact. This period of time is indicative of the integrity of the tooth interface.
There are benefits to the Periotest system. The Periotest handpiece provides a convenient means to dynamically excite the implant abutment system in areas that may be too cramped to utilise Osstell or impact hammer devices. Also, the Periotest handpiece can be used on implant abutment systems with non-recoverable, cemented restorations. As well, the output signal from the accelerometer may contain information unavailable to the RFA systems, which can be more completely utilised to determine the status of the interface layer. For example, the handpiece has recently been adapted for use in a system designed to measure the damping capacity of materials.
Several researchers have attempted to adopt the Periotest in monitoring the integrity of artificial implants instead of natural teeth. The results of these investigations have shown varying degrees of success. When used to monitor the mobility of natural teeth, the contact time is not used directly but is used to calculate a so-called Periotest value (PTV) which was originally chosen to correspond to the established Miller Mobility Index for natural teeth. For natural teeth, which are supported by periodontal ligaments, the PTV's range is from approximately −8 to 50 with −8 representing a tooth with a very stiff supporting structure and a PTV of 50 corresponds to a tooth which is noticeably loose and moveable by finger pressure.
When used to measure artificial implants the contact times involved correspond to PTV's that are significantly lower than for natural teeth, as the bone to implant interface provides a much stiffer supporting structure than periodontal ligaments. Since the Periotest has a built in lower PTV limit of −8 and only produces integer values, there is a limited range of PTV readings available for a typical implant application. For example, it has been found that well integrated implants have a range of PTV values between −7 and 0 in the mandible and −7 to +1 in the maxilla at the time of abutment connection. This limited range does not provide enough resolution to monitor subtle changes in the bone-implant interface over time. The Periotest system cannot accurately determine a contact time for very stiff implant interfaces, especially for those that are extraoral.